ChemTexts (2019) 5:12 https://doi.org/10.1007/s40828-019-0086-3

LECTURE TEXT

Intriguing minerals: lorandite, ­TlAsS2, a geochemical detector of solar neutrinos

Gligor Jovanovski1,2 · Blažo Boev3 · Petre Makreski2 · Trajče Staflov2 · Ivan Boev3

Received: 1 April 2019 / Accepted: 15 April 2019 © Springer Nature Switzerland AG 2019

Abstract This lecture text demonstrates how the mineral lorandite is used as a detector of solar neutrinos. Because the age of the lorandite deposit in Allchar, North Macedonia, is known, this opened the possibility to determine the solar neutrino fux over the last 4.3 million years.

Keywords Lorandite · Thallium · Lead · Mineral · Neutrino

Lorandite darker red color, semimetallic luster and perfect cleavage along the (001), (201) and (110) planes. Some crystals are Lorandite (or lorándite), ­TlAsS2, is well known as the most coated with a brownish yellow crust. Lorandite crystals common thallium-containing mineral [1, 2]. It was frst of 1 cm are typical for this site, although exceptional sin- discovered in the Allchar mine, Macedonia, in 1894 [3–5], gle crystals up to 5 cm have also been found. Lorandite is and the frst chemical analysis was performed by Loczka named in honor of the Hungarian physicist Loránd Eötvös [6]. Smaller quantities were later found in other localities (1848–1919) (Fig. 1). worldwide: the Dzhizhikrut Sb–Hg mine in Tajikistan, the The Allchar locality has been known since the twelfth Beshtau uranium deposit in Russia, the Lanmuchang Hg–Tl or thirteenth century, and according to other estimations, deposit in China, the Zarshuran gold mine in Iran, the Len- even longer. The chemical composition of lorandite from genbach Quarry in Switzerland, and three locations in the Allchar was confrmed by Jannasch [9] soon after the frst USA (New Rambler Cu–Ni mine in Wyoming, Jerritt Can- analysis conducted by Loczka [6]. Allchar’s lorandite is a yon mines in Nevada, and Mercur gold mine in Utah) [2]. pure mineral, containing only traces of K, Cr, Fe, Cu, Pb Nevertheless, Allchar remains the richest lorandite-bearing and Zn [10–16]. The ore-grade in the richest zone contains locality in the world. about 18,000 m­ 3 of the ore, with an average thallium content The monoclinic tabular aggregates of lorandite are typi- of 0.35%. Lorandite is the most heavily exploited sulfosalt cally dispersed throughout the realgar (As­ 4S4) and orpiment mineral from Allchar [17–19] because of its ability to act as (As­ 2S3) mineral hosts. Well-developed crystals are very rare; a geochemical detector of the solar proton–proton (pp) neu- however, 32 unique crystal forms have been observed [4, trino fux [20, 21]. Neutrinos are produced in vast numbers 7, 8]. Lorandite is easily distinguished from realgar by its by the Sun as a result of ongoing fusion processes taking place in the Sun’s interior. It is known that the Sun generates most of its energy via the pp chain, which is responsible for * Gligor Jovanovski 98.4% of the solar output [22]. In the frst reaction of the pp [email protected] chain, a proton decays into a neutron in the immediate vicin- 1 Research Center for Environment and Materials, Academy ity of another proton. The two particles form the hydrogen of Sciences and Arts of North Macedonia, MASA, Bul. isotope deuterium, along with a positron and an electron Krste Misirkov 2, 1000 Skopje, North Macedonia neutrino (Eq. 1): 2 Institute of Chemistry, Faculty of Natural Sciences → 2 + and Mathematics, Ss. Cyril and Methodius University, p + p H + e + e. (1) 1000 Skopje, North Macedonia 3 Faculty of Natural and Technical Sciences, Goce Delčev University, 2000 Štip, North Macedonia

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̄� of the neutrino (more correctly electron antineutrino e ) had been seen earlier in recoil experiments [30]. In 1956, the American physicists Clyde Lorrain Cowan Jr. (1919–1974) and Frederick Reines (1918–1998), in collaboration with other scientists [31, 32], discovered the neutrino according to the reaction ̄� → + p + e n + e . (4) In recognition of this discovery, Frederick Reines was awarded the 1995 Nobel Prize in Physics, which he shared with the American chemical engineer and physicist Martin Lewis Perl (1927–2014), awarded for his discovery of the tau lepton. The Sun is a powerful source of neutrinos, which are the Fig. 1 Lorandite from Allchar product of nuclear fusion reactions taking place in its core 10 −2 −1 (Eq. 1), the total fux of which is Φν = 6.5 × 10 ν cm s Solar neutrino detection [33]. The basis of solar neutrino detection is the fact that the dominant component in the solar neutrino spectrum consists Interest in the Allchar ore locality has increased consider- of so-called pp-neutrinos, whose fux can be determined by ably since 1976, when the American physicist Melvin Freed- means of radiochemical and geochemical detectors [34]. man (1930–1997) realized that the mineral lorandite could Neutrinos are neutral (uncharged) particles with a mass be used as a geochemical dosimeter for the solar neutrino very close to zero. They possess incredible penetration abil- fux. The idea is simple: the Allchar ore locality is the larg- ity, and travel at nearly the speed of light, from the Sun’s to est lorandite deposit in the world, and its age is known to be the Earth’s surface in around 8 min. Therefore, practically 4.31(2) Ma [23]. Since 205Tl interacts with neutrinos accord- all neutrinos that strike the Earth penetrate to its opposite ing to side. However, an exceptionally small number interact with condensed matter on Earth. In order to count the neutrinos 205 ≥ →205 − Tl + e(E 52 keV) Pb + e , (2) and then to confrm the standard solar model (SSM) pro- the average neutrino fux Φν from the Sun can be calculated posed by Bahcall [22], which predicts that most of the fux using the equation: comes from the pp-neutrinos with energies below 0.4 MeV, the American chemist and physicist Raymond (“Ray”) Davis N205Pb, exp − N205Pb, B Jr. (1914–2006) in 1960 [35, 36] placed 100,000 gallons  = C . t (3) m 1 − e  (= 379 cubic meters) of tetrachloroethylene, ­C2Cl4, at a depth of about 1500 m in a gold mine in the US state of −19 −1 C = 3.79 × 10 mol ­a , σν: cross section of solar pp- South Dakota. The depth is crucial because it is necessary 205 neutrino capture by Tl, N205Pb, exp : number of experimen- to eliminate the cosmic radiation efect, which could be 205 tally determined Pb atoms per mass m, N205Pb, B : number of registered by the detector, causing inaccurate experimen- 37 205Pb atoms per mass m formed by background reaction, λ: tal results. The precise measurements ( Ar produced in the 37 decay constant of 205Pb (λ = 4.01(16) × 10−8 ­a−1), t = 4.31(2) reaction between neutrino particles and Cl) lasted about Ma [2, 24, 25]. In age determinations [26, 27] using the 20 years. Davis confrmed the functionality of the detector, formation and decay of radioisotopes, the activation fux is but the number of neutrinos registered amounted to only known and the activation time is calculated, whereas in this one-third of the theoretical prediction. This result, which case the age is known and the fux is calculated. became known as “the solar neutrino problem”, confounded Neutrinos were frst introduced to physics as hypothetical physicists for years. Two alternatives were considered: modi- particles by the Austrian-born theoretical physicist Wolf- fying the model or checking the experiments. As a conse- gang Ernst Pauli (1900–1958) [28]. At that time, Pauli had quence of the unexpected and possibly inaccurate results, to cope with the fact that electrons, emitted in ordinary beta various alternative solar neutrino detectors using gallium, decays, exhibited a continuous spectrum, suggesting a three- lithium, manganese and other elements were considered for body rather than a two-body decay. The third particle, which future research. had been postulated and initially named “neutron” by Pauli, To that end, in 1975, the Argonne National Labora- was given its current name by the Italian-born American tory in Chicago, Illinois sent a letter to the Yugoslavian physicist Enrico Fermi (1901–1954) [29], who called it government, requesting information regarding the condi- “neutrino”, meaning “little neutron” in Italian. The existence tion of the Allchar deposit, which was known to have the

1 3 ChemTexts (2019) 5:12 Page 3 of 5 12 richest concentration of thallium minerals in the world. After [40]. The main purpose was to evaluate the erosion around receiving an answer from the Yugoslavian government, Pro- the mine during the period of over four million years of fessor Melvin Freedman from Argonne National Laboratory existence of lorandite, and to calculate the input of both cos- visited the Fe–Ni mining company at Kavadarci and All- mic radiation and natural radioactivity during that period. In char. Freedman et al. [24] proposed thallium as a detector of the meantime, all projects based on the use of geochemical neutrino activity by measuring the reaction with neutrinos, detectors for counting neutrinos were interrupted. In order to which transform thallium into lead. promote the project, two international conferences and fve According to Freedman et al. [24], solar neutrino detec- international workshops were organized. tion was possible based on these rare interactions. The The problem of solar neutrinos has since been solved by a authors proposed that one of the thallium isotopes (205Tl) series of experiments performed at several places around the in the mineral lorandite, which during the interaction with world. The most important of these lasted around 20 years neutrinos transforms into the lead isotope 205Pb (cf. Eq. 2), and were performed at the Institute for Cosmic Ray Research could be used as neutrino detector. Their idea was to analyze at the Kamioka Observatory in Japan [41]. Similar experi- thallium-containing lorandite from Allchar and examine the ments were performed at the Sudbury Neutrino Observa- quantity of transformed 205Tl into 205Pb. This quantity of tory located at the mines in Sudbury, Canada, at a depth 205Pb is a basis for calculating the number of neutrinos which of around 2000 m [42]. All of these long-range, expensive over the millennia have passed through lorandite, enabling experiments have shown that Davis’ measurements were suf- the calculation of the Sun’s fux that could help in determin- fciently correct. These fundamental investigations are still ing the Sun’s age according to SSM [37]. Freedman et al. in progress, with the fnal aim of constructing more sensible [24] considered that by analyzing old thallium minerals, the detectors for solar neutrinos. quantity of lead determined should be equivalent to the num- From theories for detecting neutrinos to trapping suf- ber of neutrinos which have passed through the mineral over fcient numbers of this mysterious, almost massless parti- a period of about fve million years, enabling the estimation cle over a period of almost 50 years, the heads of both the of total solar activity. The father of the model, the American Homestake experiment and the Super-Kamiokande Neu- astrophysicist John Norris Bahcall (1934–2005) [22, 38], at trino Detection Experiment, Raymond Davis Jr. and Japa- frst expressed skepticism regarding Freedman’s hypothesis. nese physicist Masatoshi Koshiba, were awarded the Nobel His reservations about the geochemical detectors, among Prize in Physics in 2002, “for pioneering contributions to others, were related to the erosion that occurred at the site astrophysics, in particular for the detection of cosmic neu- over millions of years, which would make it impossible to trinos”. Only one-third of John Bahcall’s calculated rate at measure the role of cosmic radiation and the participation of which the detector should capture neutrinos was found. This the background radioactivity. However, Freedman’s hypoth- signifcant discrepancy was eventually explained by neutrino esis was accepted in 1983 by the German scientists (led by oscillations, a concept once proposed by the Italian nuclear Professors H. Morinaga and E. Nolte from the Technical physicist Bruno Pontecorvo (1913–1993) [43]. University of Munich), and the locality of Allchar, with its Allchar is a remarkably interesting and rare mineralogical relatively high quantities of lorandite, was chosen for con- ore deposit—exploited during certain periods in the past for ducting the investigations of solar neutrino detection. its ore, but “exploited” at the end of the previous century for The driving force behind the LOREX project (LORandite fundamental investigations in physics and chemistry [2]. Its EXperiment) of geochemical detection of solar neutrinos has importance as a source of ore reserves is well illustrated been Miodrag Pavićević, professor of physical chemistry at by the fact that in 2009, the Parliament of the Republic of the Faculty of Mining and Geology, University of Belgrade. Macedonia declared the Allchar locality a natural monument Since 1985, the project has been fnanced by federal govern- by a special law [44]. This legislation designated the Allchar ment funds. Scientists from several institutions in Macedo- locality as a protected zone where scientifc investigations nia have also participated in the project. New excavations at could be undertaken only with special permission from the Allchar’s locality were started at a depth of around 30 m. By Ministry of Environment and Physical Planning, although careful separation of about 10 tons of ore, 1 kg of lorandite ecotourism was mentioned as a unique type of permitted was obtained at a steel factory in Skopje [39, 40]. economic activity. However, in 2012, at the proposal of the The project was fnanced by various funds until 2000. Ministry of Environment and Physical Planning, the Parlia- The follow-up fnancing of the German-Yugoslavian project, ment of the Republic of Macedonia rescinded the decla- however, was not approved by the European Union. Miodrag ration of the Allchar locality as a natural monument [45]. Pavićević is now an honorary professor at the University of According to a new law, geological explorations to ascer- Salzburg; the project was fnanced by the Austrian Govern- tain the presence of economically important quantities of ment from 2008 to 2010 and was further extended from 2012 gold, thallium and antimony are permitted, as well as the to 2015, coordinated by professors Pavićević and Amthauer continuation of the project activities. The real cause for the

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